MORPHOMETRIC ANALYSIS OF THE EVOLUTIONARY PATTERNS OF SIDERASTREA DURING THE MIO-PLIOCENE OF THE DOMINICAN REPUBLIC

Gould (1972) proposed the theory of punctuated equilibrium as an alternative to Darwin's theory of phyletic gradualism. Since Gould's original proposal, few examples of punctuated equilibrium have been demonstrated. Cheetham (1986) and Nehm and Geary (1994) are two exceptions. There has never been a quantitative study of the evolutionary patterns in corals in which samples were collected within one long continuous sequence to determine if species exhibit stasis or gradual evolutionary change. We perform such a test using reef corals in the fossiliferous sequence of the Neogene of the northern Dominican Republic. Samples of the genus Siderastrea were taken from four formations (Cercado, Gurabo, Mao, Baitoa) in the stratigraphic sequence, using sections previously constructed and dated by Saunders et al. (1986), covering approximately a 5 million year period during the Mio-Pliocene. An average of 15 samples were collected per formation. 63 corals were thin sectioned and analyzed using geometric morphometrics. 28 landmarks were used to capture differences between corals in the structure of the synapticulothecal wall associated with primary and secondary septa. This shape variation was then analyzed statistically using multivariate analyses to distinguish species and patterns of variation within species. These multivariate analyses included Canonical Discriminate Analysis, Principal Component Analysis and cluster analysis. To guide in species discrimination, thin-sections of modern species and types were also measured and analyzed. Three previously known species of Siderastrea and one new species were found to have very little morphologic overlap and to exhibit no change throughout the five million year period studied. These results support the theory of punctuated equilibrium.